Carbon in Red Giants in Globular Clusters and Dwarf Spheroidal Galaxies

Abstract

We present carbon abundances of red giants in Milky Way globular clusters and dwarf spheroidal galaxies (dSphs). Our sample includes measurements of carbon abundances for 154 giants in the clusters NGC 2419, M68, and M15 and 398 giants in the dSphs Sculptor, Fornax, Ursa Minor, and Draco. This sample doubles the number of dSph stars with measurements of [C/Fe]. The [C/Fe] ratio in the clusters decreases with increasing luminosity above log(L/L ⊙ ) ≃ 1.6, which can be explained by deep mixing in evolved giants. The same decrease is observed in dSphs, but the initial [C/Fe] of the dSph giants is not uniform. Stars in dSphs at lower metallicities have larger [C/Fe] ratios. We hypothesize that [C/Fe] (corrected to the initial carbon abundance) declines with increasing [Fe/H] due to the metallicity dependence of the carbon yield of asymptotic giant branch stars and due to the increasing importance of Type Ia supernovae at higher metallicities. We also identified 11 very carbon-rich giants (8 previously known) in three dSphs. However, our selection biases preclude a detailed comparison to the carbon-enhanced fraction of the Milky Way stellar halo. Nonetheless, the stars with [C/Fe] < +1 in dSphs follow a different [C/Fe] track with [Fe/H] than the halo stars. Specifically, [C/Fe] in dSphs begins to decline at lower [Fe/H] than in the halo. The difference in the metallicity of the [C/Fe] "knee" adds to the evidence from [α/Fe] distributions that the progenitors of the halo had a shorter timescale for chemical enrichment than the surviving dSphs.

The early days of the Sculptor dwarf spheroidal galaxy

“…The cyan circles show the observations in Draco by Shetrone et al (2013) and Kirby et al (2015), while the point above the dotted line is the carbon-rich RGB found by Cohen & Huang (2009) in this galaxy. The observations of Kirby et al (2015) in Ursa Minor are shown in purple. The references of the other comparisons samples are given in Sect.…”

“…We indicate in Fig. 7 the Draco stars with cyan circles (Cohen & Huang 2009;Shetrone et al 2013;Kirby et al 2015).…”

Portrait of a Dark Horse: A Photometric and Spectroscopic Study of the Ultra-Faint Milky Way Satellite Pegasus Iii*

“…The red star #2 of Peg III can indeed be fitted with an isochrone for a higher metallicity of [Fe/H]=−1.1 (dashed curve in the middle panel) at the same distance modulus. Carbon stars ([Ca/Fe] +  1.0) in dwarf galaxies also tend to be redder than carbon-normal RGB stars due to the Bond-Neff effect (Bond & Neff 1969), as shown for instance in Figure 7 of Kirby et al (2015b). The possibility of a metallicity and carbon spread among the Peg III stars can be tested once the information on the chemical abundances of the individual stars becomes available.…”

The Next Generation Virgo Cluster Survey Xvi: The Angular Momentum of Dwarf Early-Type Galaxies From Globular Cluster Satellites

“…In Table 3, we summarize the observations. The raw two-dimensional spectra are reduced and extracted into one-dimensional spectra using the standard SPEC2D pipeline for DEIMOS designed by the DEEP Galaxy Redshift Survey team (Cooper et al 2012;Newman et al 2013) and modified by Simon & Geha (2007) to optimize the reduction of unresolved targets and by Kirby et al (2015) to make it more suitable for bluer DEIMOS spectra, given that the standard DEIMOS pipeline was designed for the 1200l/mm grating. The main steps in the reduction process consist of flat-field and fringe corrections, wavelength calibration, sky subtraction, and cosmic-ray cleaning.…”